Apparatus for producing oriented plastic strap

ABSTRACT

An apparatus for producing an oriented plastic strap having a predetermined desired thickness and flatness characteristics for use in connection with the strapping of packages or the like, is disclosed. A sheet workpiece is conducted into a nip defined between a pair of opposed milling and stretching rollers wherein one of the rollers is rotated at a lineal surface velocity which is greater than the lineal surface velocity of the other roller, and the rollers are rotated in opposite directions. One of the rollers effectively brakes or retards one of the surfaces of the sheet workpiece while the other one of the rollers effectively accelerates the other one of the surfaces of the sheet workpiece as the workpiece passes through the nip defined between the rollers whereby the workpiece is simultaneously milled and stretched. The workpiece is heated along its side portions disposed inwardly from the side edge portions so as to eliminate concavities and bump transitional points which normally tend to form at and within the vicinity of the side portions, and in addition, the side edge portions of the sheet workpiece are also heated so as to cooperate with the heated, axially inward side portions so as to advantageously control the degree of flatness of the sheet workpiece across the width thereof after the same has been milled and stretched.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a divisional application of 08/474,355 filedJun. 7, 1995 which is Continuation-In-Part (CIP) patent application ofU.S. patent application Ser. No. 08/353,721 filed on Dec. 12, 1994 byDonald L. Van Erden et al. and entitled METHOD AND APPARATUS FORPRODUCING ORIENTED PLASTIC STRAP, AND STRAP PRODUCED THEREBY now U.S.Pat. No. 5,525,287 which, in turn, is a Continuation-In-Part (CIP)patent application of U.S. patent application Ser. No. 07/958,803 filedon Oct. 9, 1992 by Donald L. Van Erden et al. and entitled METHOD ANDAPPARATUS FOR PRODUCING ORIENTED PLASTIC STRAP, now U.S. Pat. No.5,387,388.

FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus forproducing an oriented plastic strap, and a strap produced by such methodand apparatus, and more particularly to a method and apparatus forsimultaneously milling and stretching a plastic sheet into strap stockmaterial having a predetermined desired thickness profile.

BACKGROUND OF THE INVENTION

In accordance with a typical prior art stretching process, such as, forexample, the SIGNODE process, a cast sheet of thermoplastic material,such as, for example, polypropylene, is first reduced in size, that is,the thickness dimension thereof, by rolling the sheet through a pair ofclosely spaced milling rollers or cylinders which rotate in oppositedirections. After the thickness of the sheet has been reduced, the sheetis then drawn and stretched out from the milling rollers by means of aseries of orienting rollers or a bridle assembly so as to achieve itsfinalized desired size or thickness.

Another prior art process or method that is commonly used in connectionwith the fabrication of stretched sheet materials is a process known asor called the short-gap method and is generally comprised of an entrybridle, a stretching assembly, and an exit bridle. In accordance withthis process, a slow speed, heated entry bridle assembly advances a castsheet of material, usually film, to a stretching assembly whichcomprises a pair of rollers or cylinders which are set a predetermineddistance apart. The first roller rotates at the same speed as the entrybridle, whereas the second roller rotates at a speed which is greaterthan that of the first roller and which is equal to the rotary speed ofthe exit bridle. Thus, as the film passes through the entire assembly,it is stretched to its finalized desired size or thickness dimension.

These prior art methods or processes have presented severaldisadvantages. For example, the properties of the straps produced bythese methods or processes provide or exhibit limited increases instrength without significant decreases in other desired properties. Inaddition, substantial necking of the sheets occur as the sheets arestretched over the distance or space defined between the rollers.

U.S. patent application Ser. No. 07/958,803, now U.S. Pat. No.5,387,388, disclosed a novel process and apparatus for achieving thesimultaneous milling and stretching of such sheets whereby theaforenoted problems were minimized, however, other operational problemswith such process and apparatus were discovered which likewise requiredrectification in order to achieve simultaneously milled and stretchedsheets of thermoplastic material which exhibited other desirableproperties. For example, one operational problem or deficiency which wasdiscovered in connection with the fabrication or manufacture of thesimultaneously milled and stretched sheets of thermoplastic material inaccordance with the zero-gap process and apparatus set forth anddescribed within U.S. Pat. No. 5,387,388 is that when the thermoplasticsheet is ready to be worked, that is, simultaneously milled andstretched, by being conducted through the nip defined between theoppositely rotating rollers of the zero-gap assembly, it was found thatthe opposite surfaces of the thermoplastic sheet exhibited differentsurface temperature values. This is because as the thermoplastic sheetis routed about the first or upstream roller of the zero-gap rollerassembly, the inside surface of the thermoplastic sheet that is, thesurface of the sheet which is disposed in direct contact with the firstor upstream roller of the roller assembly, will be heated by the firstor upstream roller, or at least have its surface effectively preventedfrom being cooled by the ambient air because such surface is disposed indirect contact with the the first or upstream roller and not exposeddirectly to the ambient air. On the contrary, the outer or exteriorsurface of the thermoplastic sheet, that is, the surface of the sheetwhich is not disposed in direct contact with the first or upstreamroller, is effectively cooled as a result of being exposed to theambient air. Such temperature differential between the aforenotedsurfaces of the thermoplastic sheet leads to density differentialsthroughout the thermoplastic sheet which adversely affects the variousproperties of the processed sheet, such as, for example, the tensilestrength, weldability and associated properties, and split resistance.

Another problem or deficiency which was discovered in connection withthe aforenoted zero-gap simultaneous milling and stretching apparatusand method was that as a result of the processing of the thermoplasticsheet, the opposite edge portions of the sheet became thickened, or inother words, the sheet did not exhibit uniform thickness or flatnessacross the width thereof. Consequently, when the processed sheet is tobe subsequently processed into thermoplastic strapping, the thickenededge portions cannot be used to fabricate such straps unless furtherprocessing is performed upon the sheet so as to effectively reduce thethickness dimension of the edge portion to the desired thicknessdimension characteristic of useable strapping.

Accordingly, the invention embodied and disclosed within U.S. patentapplication Ser. No. 08/352,721 was directed toward an apparatus andmethod for producing an oriented plastic strap which was simultaneouslymilled and stretched as a result of a thermoplastic sheet workpiecebeing passed through a nip defined between the oppositely rotatingrollers of the zero-gap assembly, and wherein, in particular, in orderto overcome the aforenoted deficiency of the zero-gap simultaneousmilling and stretching apparatus and method with respect to thethickened edge portions of the milled and stretched sheet, edge heaterswere disposed along the opposite edge regions of the sheet workpieceprior to entry of the sheet workpiece into the nip defined between thezero-gap milling rollers. Such edge heating was found to significantlyreduce the width of the thickened edge portions of the milled andstretched sheet, as measured inwardly from the edge portions of thesheet, such that an increased amount of useable sheet could be producedwhile concomitantly resulting in a reduced amount of sheeting which hadto be previously trimmed and discarded.

Additional studies performed in connection with the strap profile andthe effects upon the same by means of the disposition of the aforenotededge heaters have revealed, however, that while such edge heaters havein fact reduced the thickness dimensions of the sheets at the extremeouter edge portions thereof, concave portions or concavities arenevertheless still present within regions immediately adjacent to theextreme outer edge portions of the sheets. The concave portions havedepth or thickness dimensions which are somewhat less than the thicknessdimension characteristic of the main central portion of the milled andstretched sheet. In addition, a transitional bump region is also presentat the junction between the main central portion of the sheet and theconcave side edge portions thereof, and such transitional bump regionshave thickness dimensions which are greater than that of the maincentral portion of the sheet.

A need therefore exists for the development of apparatus, and aconcomitant method, for producing oriented plastic sheets bysimultaneously milling and stretching the same wherein the producedsheets exhibit substantially uniform thickness dimensions across theentire width thereof from one edge portion to the opposite edge portionby effectively eliminating the aforenoted concavities and transitionalbump regions.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide a newand improved method and apparatus for producing oriented plastic strap,and an oriented plastic strap produced by such method and apparatus.

Another object of the present invention is to provide a new and improvedmethod and apparatus for producing oriented plastic strap, and anoriented plastic strap produced by such method and apparatus, whichovercomes the various disadvantages, problems, and deficiencies of theprior art processes and apparatus.

Yet another object of the present invention is to provide a new andimproved method and apparatus for producing oriented plastic strap whichis capable of simultaneously milling and stretching a sheet workpieceinto a strap which has a predetermined thickness dimension as a resultof the sheet workpiece being conducted in a single pass through a nipdefined between a pair of zero-gap rollers.

Still another object of the present invention is to provide a new andimproved method and apparatus for producing oriented plastic strap, andan oriented plastic strap produced by such method and apparatus, whichhas significantly increased tensile strength and split resistance whencompared to straps produced by heretofore known prior art methods andapparatus.

A further object of the present invention is to provide a new andimproved method and apparatus for producing oriented plastic strap, andan oriented plastic strap produced by such method and apparatus, whichexhibits high tensile strength, high split resistance, and improvedwelding characteristics.

A yet further object of the present invention is to provide a new andimproved method and apparatus for producing oriented plastic strap, andan oriented plastic strap produced by such method and apparatus, whereinuniform density characteristics are imparted to the sheet workpiece suchthat the milled and stretched strap produced within the zero-gap rollerassembly exhibits improved tensile strength, weld, and split resistanceproperties.

A still further object of the present invention is to provide a new andimproved method and apparatus for producing oriented plastic strap, andan oriented plastic strap produced by such method and apparatus, whereinimproved uniform thickness and flatness characteristics are imparted tothe finalized milled and stretched strap as a result of various heatingtechniques being imparted to the original sheet workpiece or material.

SUMMARY OF THE INVENTION

Briefly, the foregoing and other objects are achieved in accordance withthe present invention through the provision of a method and apparatusfor producing an oriented plastic strap which will have a predetermineddesired thickness for use in strapping packages and the like, whereinthe strap, having the noted predetermined desired thickness, issimultaneously milled and stretched as a result of a plastic sheetworkpiece being passed through a nip defined between a pair of rollersor cylinders which are spaced closely together with respect to eachother. The nip has a space dimension which is substantially less thanthe original thickness dimension of the sheet workpiece, and the rollersare rotated in opposite directions and at substantially different linealsurface velocities.

In order to improve the various properties of the simultaneously milledand stretched strap, the density of the sheet workpiece, prior to entryinto the aforenoted nip defined between the pair of milling rollers, isrendered more uniform across the thickness of the sheet workpiece byheating the outer surface of the sheet workpiece to an elevatedtemperature level or value, that is, to a temperature level or valuewhich is greater than that of the inner surface of the sheet workpiecewhich is disposed in contact with the first or upstream one of themilling rollers, such that despite the cooling of the outer surface ofthe sheet workpiece as a result of the exposure of the outer surface ofthe sheet workpiece to the ambient air, the inner and outer surfaceswill exhibit substantially the same temperature levels and thereforesubstantially the same or uniform density values.

In order to improve the flatness of the resultant simultaneously milledand stretched sheet such that increased volumetric strapping can beproduced from such sheeting, edge heaters are disposed along theopposite edge regions of the sheet workpiece prior to entry of the sheetworkpiece into the nip defined between the zero-gap milling andstretching rollers. Such edge heating has been found to significantlyreduce the width of thickened edge portions of the milled and stretchedsheet, as measured inwardly from the edge portions of the sheet, suchthat an increased amount of useable sheet can be produced whileconcomitantly resulting in a reduced amount of sheeting which must betrimmed and discarded.

Still further, in order to further improve the flatness of the resultantsimultaneously milled and stretched sheet such that increased volumetricstrapping can be produced from such sheeting, strip heaters are disposedaxially inwardly with respect to the edge heaters as considered orviewed along the longitudinal axes of the milling and stretchingrollers. The strip heaters are disposed about the first or upstream oneof the milling and stretching rollers and at positions prior to orupstream of the nip defined between the zero-gap milling and stretchingrollers. The strip heaters can be employed alone, that is, withoutconjunctive use in connection with the aforenoted edge heaters, orpreferably, the strip heaters can be employed with and supplemental tothe aforenoted edge heaters. The use of such strip heaters, particularlyin conjunction with the aforenoted edge heaters, has been found tosubstantially eliminate the aforenoted concave portions of thesimultaneously milled and stretched sheeting, and similarly, tosignificantly reduce the thickness dimensions of the aforenotedtransitional bump regions defined between the main central portion ofthe sheeting and the concave edge portions of the sheeting.Consequently, the sheeting is provided with a substantially uniformthickness dimension or profile across the entire width thereof extendingbetween one side edge and the opposite side edge whereby an increasedvolumetric amount of useable sheeting can be produced and is availablefor finalized production or fabrication of the desired plasticstrapping.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and attendant advantages of the presentinvention will be more fully appreciated from the following detaileddescription when considered in connection with the accompanying drawingsin which like reference characters designate like or corresponding partsthroughout the several views, and wherein:

FIG. 1 is a fragmentary, front view of apparatus, constructed inaccordance with the present invention, for producing oriented plasticstrap in accordance with the zero-gap manufacturing process of thepresent invention;

FIG. 2 is an enlarged, simplified, fragmentary front view of the millingand stretching rollers defining or comprising the zero-gap assembly ofFIG. 1;

FIG. 3 is an enlarged, partial, cross-sectional view of the milling andstretching rollers defining the zero-gap assembly of FIG. 1 as takenalong the line 3--3 of FIG. 1;

FIG. 4 is a simplified, fragmentary, cross-sectional view, on a reducedscale, of the milling and stretching rollers defining the zero-gapassembly of FIG. 1, as taken along a line or direction parallel to line3--3 of FIG. 1;

FIG. 5 is a front view of apparatus, constructed in accordance with thepresent invention and similar to the apparatus shown in FIG. 1, showing,however, a modified embodiment for producing oriented plastic strap inaccordance with the zero-gap manufacturing process of the presentinvention wherein the sheet workpiece has its external surface heatedprior to entry into the zero-gap roller assembly, as well as its edgeportions heated prior to entry into the nip defined between the millingand stretching rollers of the zero-gap roller assembly;

FIG. 6 is an enlarged, simplified, fragmentary front view, similar tothat of FIG. 2, showing the milling and stretching rollers defining orcomprising the zero-gap assembly of FIG. 1 and having the edge heatersoperatively associated therewith;

FIG. 7 is an enlarged, simplified, fragmentary front view, similar tothat of FIG. 2, showing the milling and stretching rollers defining orcomprising the zero-gap assembly of FIG. 1 and having the strip heatersof the present invention operatively associated therewith;

FIG. 8 is an enlarged, simplified, cross-sectional view, similar to thatof FIG. 3, showing the relative disposition, location, or placement ofthe edge heaters and the strip heaters of the present invention withrespect to the sheet workpiece prior to, and upstream of the, entry ofthe sheet workpiece into the nip defined between the milling andstretching rollers defining or comprising the zero-gap assembly of thepresent invention; and

FIG. 9 is a graphical presentation of various cross-sectional profilesof oriented plastic strap produced in accordance with the zero-gapmanufacturing process of the present invention, wherein the variousgraphical curves or plots illustrate the effect, upon the thickness orprofile dimensions of the oriented sheets, of the strip heaters of thepresent invention when such strip heaters are used alone or inconjunction with the edge heaters, and as compared to profiles of theoriented sheet when neither edge or strip heaters are employed or whenonly edge heaters are employed.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Referring now to the drawings, and more particularly to FIG. 1 thereof,there is schematically illustrated a zero-gap roller assembly, generallyindicated by the reference character 20, for simultaneously milling andstretching or elongating a sheet workpiece 22 into a thin strap stockmaterial. The present invention is discussed and illustrated with only asingle sheet workpiece 22 being conducted through the assembly 20,however, it is to be understood that more than one sheet or workpiece 22may be simultaneously passed through the assembly 20. It is to befurther noted that the phrase or terminology "zero-gap" as usedthroughout this patent specification refers to the concept ofsubstantially eliminating any gap between the step of milling the sheetworkpiece and the step of stretching the sheet workpiece. In otherwords, the steps of milling and stretching of the sheet workpiece areaccomplished substantially simultaneously. In addition, it is noted, asdisclosed in FIG. 1, that the zero-gap assembly 20 is located between asheet or workpiece feeding assembly 24 and an exit bridle assembly 26which are located or mounted upon a frame or support 28.

The feeding assembly 24 may take any one of several forms, and as shownin FIG. 1, comprises an extruding machine 30 for extruding a sheet orworkpiece 22 of stock material, and an entry bridle assembly 32. Theextruding machine 30 produces a sheet workpiece 22 from a suitablematerial, such as, for example, polypropylene, polyethyleneterephthalate, or the like, and the sheet workpiece 22 is conducted intothe entry bridle assembly 32 which, in turn, feeds the sheet workpieceinto the zero-gap roller assembly 20. The sheet workpiece 22 may beheated during its passage through the entry bridle assembly 32 so as toin effect be pre-heated upon entry into the zero-gap assembly 20 inorder to enhance the working properties of the material comprising thesheet workpiece 22.

The entry bridle assembly 32 is seen to comprise a plurality of rollersor cylinders 34, 36, 38, and 40 which are mounted by suitable means,such as, for example, shafts, not shown, upon the frame or support 28.The rollers 34, 36, 38, and 40 may be either solid or hollow, and in thepreferred embodiment, as illustrated in FIG. 1, the rollers 34, 36, 38,and 40 are essentially only used to properly deliver or feed the sheetworkpiece 22 into the zero-gap assembly 20, and they do notsubstantially contribute to any stretching or milling of the sheetworkpiece 22. A different number of rollers may be employed than thatshown in FIG. 1, and it is seen that the rollers 34, 36, 38, and 40 arearranged within two vertically spaced rows with the bottom row rollers36 and 40 being located between or offset with respect to the top rowrollers 34 and 38. Rollers 34 and 38 are also mounted for rotation inthe clockwise direction while rollers 36 and 40 are mounted for rotationin the counterclockwise direction, whereby when the sheet workpiece 22is wound around or routed through the entry bridle assembly 32, ittravels through the rollers 34, 36, 38, and 40 in the proper mode ordirection with respect to the directions of rotation of the rollers 34,36, 38, and 40. Each one of the rollers 34, 36, 38, and 40 are rotatedat a uniform speed by suitable means, not shown, such as, for example, amotor and shaft drive assembly, and all of the rollers 34, 36, 38, and40 rotate at substantially the same speed or lineal surface velocity asthe top roller 42 of the zero-gap assembly 20, which will be discussedin greater detail hereinafter.

Continuing further, after the sheet workpiece 22 passes through thefeeding assembly 24, it advances toward the zero-gap assembly 20 forsimultaneous milling and stretching into a finished sheet 22 which has apredetermined desired thickness. The zero-gap assembly 20 comprises apair of rollers or cylinders 42 and 44 that are rotatably mounted in anopposing relationship. The nip 46, that is, the distance defined betweenthe rollers 42 and 44, can be substantially varied depending upon thedesired finished thickness of the finished sheet 22. The zero-gaprollers 42 and 44 may either be solid or hollow, and may be heated byany well-known means, not shown, such as, for example, circulating aheated fluid through the rollers 42 and 44, in order to enhance thestretching properties of the sheet material. The zero-gap rollers 42 and44 may also be flat, as shown in FIGS. 1-4, or may be contoured, notillustrated herein but illustrated in the aforenoted patent applicationSer. No. 08/352,721, in order to change the shape of the sheet workpiece22 as it passes through the rollers 42 and 44 of the zero-gap assembly20.

As best shown in FIG. 2, the upper roller 42 is driven in a clockwisedirection, as shown by means of its arrow, and the bottom or lowerroller 44 is driven in the counterclockwise direction as similarly shownby means of its arrow. Thus, the sheet workpiece 22 is first routedaround a major circumferential portion of the top or upper roller 42,the workpiece 22 is then conducted through the nip 46 defined betweenthe rollers 42 and 44, and is then conducted around a majorcircumferential portion of the bottom or lower roller 44. Moreparticularly, it is appreciated that the sheet workpiece 22 is disposedin surface contact with each one of the rollers 42 and 44 over acircumferential extent which is greater than one-half of each of thecircumferential dimensions of each one of the rollers 42 and 44, and itis further noted that as a result of this particular routing of thesheet workpiece 22 through the nip 46 defined between the rollers 42 and44, and about the outer circumferential or peripheral surfaces of therollers 42 and 44, each roller 42 and 44 is disposed in contact with anopposite surface or side of the sheet workpiece 22.

In accordance with the preferred, illustrated embodiment of the presentinvention, as shown in FIGS. 1-4, and as has been described in detail,the milling and stretching rollers 42 and 44 of the zero-gap assembly 20are disposed in a top-bottom or upper-lower arrangement with respect toeach other, however, it is to be appreciated that the rollers 42 and 44may alternatively be disposed in a side-by-side arrangement in whichcase the top or upper roller 42 will be disposed toward the left of thebottom or lower roller 44 such that the roller 42 will be the firstroller that the sheet workpiece 22 contacts, while the bottom or lowerroller 44, which is now, in effect, the right roller, will be the secondroller that the sheet workpiece 22 contacts. As best seen in FIG. 2, therollers 42 and 44 are respectively mounted upon rotary shafts or axles52 and 54.

With particular reference being made to FIG. 4, the drive system for therollers 42 and 44 is disclosed therein, and it is seen that the axles orshafts 52 and 54 of the rollers 42 and 44 are operatively connected,through means of bearings 69 and 68, to driven shafts 48 and 50.Suitable drive means 56 and 58, such as, for example, electric motors,are mounted upon the support 28 and drive the rollers 42 and 44,respectively, through means of drive shafts 60 and 62 which areconnected to the shafts 48 and 50 by means of suitable couplings 64 and66, coupling 66 preferably comprising a universal coupling for reasonsto become apparent shortly. The bottom or lower roller 44 is connectedto the support 28 through means of bearings 68 and 70, and the coupling66, and bearings 68 and 70, permit the bottom or lower roller 44 to movewith respect to the support 28 by means of actuators 72 and 74. In thismanner, the bottom or lower roller 44 is able to be moved toward or awayfrom the stationary top or upper roller 42 so as to desirably change thesize of the nip 46 defined between the rollers 42 and 44. Each one ofthe driven shafts 48 and 50 is independently driven by its separatedrive means 56 and 58, and the bottom or lower roller 44 is driven at aspeed greater than that of the top or upper roller 42. In particular, inaccordance with the teachings of the present invention, the lower orbottom roller 44 is driven in such a manner that its lineal surfacevelocity is preferably within the range of seven to twelve (7-12) timesgreater than the lineal surface velocity of the top or upper roller 42when the sheet workpiece 22 comprises polypropylene, however, asdisclosed within U.S. patent application Ser. No. 08/485,538 filed Jun.7, 1995, entitled METHOD AND APPARATUS FOR PRODUCING HIGHLY ORIENTEDPOLYESTER SHEET, and filed on the same date herewith, the overall drawor lineal surface velocity ratio is approximately six (6) times greateror 6:1 when the workpiece 22 comprises polyester, that is, polyethyleneterephthalate.

In view of the foregoing, it can therefore be appreciated that as thesheet workpiece 22 is passed through the nip 46 defined between theupper and lower rollers 42 and 44 of the zero-gap assembly 20, the topor upper roller 42 effectively operates as a brake and the millreduction operation per se may also effectively operate as a brake, uponthe lower surface of the sheet workpiece 22, that is, the surfacedisposed in contact with the upper roller 42, while the bottom or lowerroller 44 pulls and accelerates the sheet workpiece 22, the roller 44effectively operating upon the upper surface of the sheet workpiece 22,that is, the surface thereof which is disposed in contact with theroller 44. As the sheet workpiece 22 accelerates through the nip 46defined between the rollers 42 and 44, it is simultaneously milled andstretched to its final predetermined thickness which may in fact be lessthan the space dimension of the nip 46 as defined between the rollers 42and 44. In particular, the thickness of the finished sheet 22 dependsupon the lineal surface velocity differential defined between the toproller 42 and the bottom roller 44, that is, the faster the bottom orlower roller 44 rotates relative to the top or upper roller 42, thethinner the finished sheet 22 will be. It is also to be noted that somestretching may occur slightly before or after the nip 46 defined betweenthe rollers 42 and 44 depending, again, upon the surface velocity of thebottom roller 44. Thus, as has been noted hereinabove, there isessentially a zero gap between the milling and stretching functions oroperations performed in connection with the sheet workpiece 22 and theprocessing thereof into the finalized or finished sheet 22. As a resultof such simultaneous milling and stretching processing, it has beennoted that there is substantially less necking of the sheet width ascompared to the parameters of sheets which have undergone stretchingmethods in accordance with prior art processes or techniques wherein thesheet is stretched only after the milling step has been completed.

After the finished sheet 22 exits from the zero-gap assembly 20, it isconducted through the exit bridle assembly 26. The exit bridle assembly26 may take any one of several different forms, and as shown in FIG. 1,the assembly preferably comprises a plurality of rollers or cylinders76, 78, 80, 82, 84, and 86 which are mounted by suitable means, such as,for example, shafts or axles, not shown, upon the support 28. Theassembly 26, and in particular the rollers 76-86, are used to pull thefinished sheet 22 out from the zero-gap assembly 20. The rollers 76, 78,80, 82, 84, and 86 may be solid or hollow, and more or fewer rollersthan those disclosed may be used. The rollers 76, 78, 80, 82, 84, and 86do not substantially contribute to any stretching of the finished sheet22, and as seen in FIG. 1, are arranged in two vertically spaced rowswith the bottom or lower row rollers 78, 82, and 86 also being spacedbetween the top or upper row rollers 76, 80, and 84 as considered in thetransverse direction. The upper rollers 76, 80, and 84 all rotate in theclockwise direction, while the lower rollers 78, 82, and 86 all rotatein the counterclockwise direction such that the finished sheet 22 can beproperly conducted through the exit bridle assembly 26. The rollers 76,78, 80, 82, 84, and 86 are rotated at a uniform rate of speed bysuitable drive means, not shown, such as, for example, a motor and shaftassembly, and the speed of the rollers 76, 78, 80, 82, 84, and 86 issuch that the lineal surface velocities thereof are essentially the sameas that of the bottom or lower roller 44 of the zero-gap assembly 20.

It is to be appreciated that in accordance with further teachings of thepresent invention, another stretching process and apparatus forimplementing the same, such as, for example, a short gap stretchingapparatus, system, or assembly, may be used either before or after thezero-gap assembly 20 so as to further modify or enhance thecharacteristics of the finished sheet 22.

Having now disclosed the specifics of the apparatus comprising thepresent invention, the method to be practiced by means of such apparatusand in accordance with the present invention will now be described. Moreparticularly, the sheet workpiece 22 is fed from the extruding machine30 to the entry bridle assembly 32 and is wound around the entry bridleassembly rollers 34, 36, 38, and 40 so as to be, in turn, properlyconducted into the zero-gap assembly 20. The sheet workpiece 22 is thenfed around the top or upper roller 42 of the zero-gap assembly 20, thetop or upper roller 42 of the assembly 20 being driven at the samelineal surface velocity as that of the entry bridle rollers 34, 36, 38,and 40. As the sheet workpiece 22 enters the zero-gap assembly 20, ittravels around the outer peripheral surface of the top or upper roller42 until it reaches the nip 46 defined between the top or upper roller42 and the bottom or lower roller 44. As previously noted, the fasterrotating bottom or lower roller 44 pulls the sheet workpiece 22 throughthe nip 46 while the slower rotating top or upper roller 42, as well asthe mill reduction process per se, effectively acts as a brake upon thelower surface of the sheet workpiece 22, that is, the surface of theworkpiece 22 which is disposed in contact with the upper or top roller42. Thus, the sheet 22 accelerates through the nip 46 defined betweenthe rollers 42 and 44, and is simultaneously milled and stretched to itsfinalized predetermined thickness dimension as the same passes throughthe nip 46. The exit bridle assembly 26 subsequently pulls the finishedsheet 22 off from the bottom or lower roller 44 of the zero-gap assembly20, and it is therefore appreciated that in accordance with the methodof the present invention, there is able to be achieved the production ofa thin, flat, oriented sheet 22 which is ready to be surface treatedand/or heat treated as desired, and sliced into thin straps as requiredor desired for use in strapping packages and the like in accordance withknown procedures.

It is again reiterated that the above-described apparatus and processproduce a significantly better quality strap than those able to beformerly produced in accordance with known or prior art apparatus andprocesses, as also exemplified and illustrated by means of the variousdata noted in the following table:

                  TABLE I                                                         ______________________________________                                                       SINGLE DRAW                                                                             ZERO-GAP                                                            PROCESS   PROCESS                                              ______________________________________                                        Tensile Strength (KPSI)                                                                        45          64                                               Elongation (%)   25          13                                               Modu1us (at 2 and 5 KPSI)                                                                      400         963                                              Weld Strength (Lbs)                                                                            79          187                                              Weld Percent     55          89                                               Weld Equivalent (KPSI)                                                                         25          57                                               Sp1it (In)       00.7        00.07                                            ______________________________________                                    

As can be appreciated from the table noted above, the zero gap methodproduces strapping which exhibits higher tensile strength, and astronger and higher percentage weld. Furthermore, splitting of thestrapping has been essentially eliminated while still achieving hightensile strength, whereas in accordance with known or prior artmanufacturing techniques and processes, as the tensile strength isincreased, increased splitting occurs and the percent of weld strengthdecreases. Still further, since the tensile strength of the strappingproduced in accordance with the zero-gap process is approximately 1.47times the tensile strength of conventionally produced strapping, andsince the elongation of such strapping of the present invention isapproximately one-half that of the conventional strapping, better creepperformance is achieved. Such results present several marketingadvantages in connection with polypropylene or polyethyleneterephthalate (PET) material straps. More particularly, if the breakstrength of the strapping is a controlling factor in connection with theparticular application or use of the strapping, then the higher tensilestrength of the material will permit the use or substitution of a strapwhich comprises only approximately seventy percent of currently usedmaterial or strapping. Similarly, if stiffness is a controlling factor,characteristic, or attribute, the strapping produced in accordance withthe present invention is such as to be readily capable of being reliablyfed into a strapping machine by being pushed around a guide chute of thestrapping machine. Still yet further, if weld strength of the strappingis a controlling factor or characteristic, then strapping comprisingless than one-half of the currently or conventionally used raw materialwill produce the equivalent joint strength.

The various properties resulting from the process of the presentinvention give significant flexibility of design to the strapping whichmay then be used in connection with a variety of applications. It isbelieved, for example, that the strapping produced by means of theprocess of the present invention exhibits a stronger bond across thegrain, while still being relatively easy to tear the strap across thegrain. Furthermore, the strap produced by means of the zero-gap processof the present invention does not exhibit structural delamination as isoften characteristic of most prior art strapping. Since the strap of thepresent invention does not exhibit structural delamination, higher weldstrength is also obtained. As has also been noted hereinbefore, if aprestretch step is performed between the entry bridle assembly and thezero-gap assembly, or if a post-stretch step is performed between thezero-gap assembly and the exit bridle assembly, the same overallcharacteristics are achieved as are achieved in accordance with theembodiment of the invention described in connection with the system andprocess of FIGS. 1-4. It is noted, however, that if a pre-stretchprocess step is performed upon the sheet workpiece 22, a higher tensilemodulus can be achieved, whereas if a post-stretch process step isperformed upon the finished sheeting 22, the material comprising thesheeting has a somewhat greater tendency toward fibrillation.

With reference again being made to the apparatus or system of FIG. 1, ithas been discovered that when the sheet workpiece 22 is conductedthrough the entry bridle assembly 32, and particularly as the workpiece22 approaches the upper milling and stretching roller 42 of the zero-gapassembly 20, the upper or outer surface of the sheet workpiece 22, thatis, the surface thereof which is not disposed in contact with theperipheral surface of the upper or top roller 42 of the zero-gapassembly 20, will tend to cool at a faster rate than the lower or innersurface of the sheet workpiece 22 which is disposed in contact with theperipheral surface of the upper or top roller 42 of the zero-gapassembly 20 in view of the fact that the upper or outer surface of thesheet workpiece 22 is directly exposed to the ambient air. As a resultof such uneven or unequal cooling of the surfaces, or as a result of thedevelopment of such a temperature differential between the notedsurfaces of the sheet workpiece 22, the sheet workpiece 22 exhibitsdensity differentials throughout the thickness thereof whereby suchdensity differentials adversely affect some of the properties orcharacteristics of the sheet workpiece. Consequently, in order torectify, resolve, or eliminate such problem, an improved system orapparatus, generally indicated by the reference character 100, has beendeveloped and is shown in FIG. 5.

The apparatus 100 is seen to comprise an entry bridle assembly 132similar to the entry bridle assembly 32 of the embodiment of FIG. 1, andsimilarly, there is disclosed a zero-gap assembly 120 which is similarto the zero-gap assembly 20 of the embodiment of FIG. 1 and is seen tocomprise upper and lower milling and stretching rollers 142 and 144,respectively. The entry bridle assembly 132 is seen to comprise rollers134, 136, 138, 140, and 141, all of which are heated to an operativetemperature of approximately 265° F. The upper or top roller 142 of thezero-gap assembly 120 is heated to a temperature of approximately 290°F., and the lower or bottom roller 144 of the zero-gap assembly 120 isheated to a temperature of approximately 250° F. In accordance with thisembodiment of the present invention, in order to maintain the outersurface of the sheet workpiece 122, that is, the surface of the sheetworkpiece 122 which is not disposed in direct contact with theperipheral or circumferential surface of the upper or top roller 142 ofthe zero-gap assembly 120, at a temperature level which is substantiallyequal to the temperature level of the inner surface of the sheetworkpiece 122, that is, the surface of the sheet workpiece 122 which isdisposed in direct contact with the peripheral or circumferentialsurface of the upper or top roller 142 of the zero-gap assembly 120, anadditional heated roller 143 is interposed between the last roller 141of the entry bridle assembly 132 and the upper or top roller 142 of thezero-gap assembly 120. Roller 143 is heated to an operative temperatureof approximately 280° F., and accordingly, it is noted that while thesurface temperature of the sheet workpiece 122 at a location just priorto entry into the entry bridle assembly 132, that is, at a position justimmediately upstream of the first entry bridle assembly roller 134, isapproximately 65° F., the surface temperature of the sheet workpiece 122at a location immediately downstream of the entry bridle assembly 132or, in other words, at the location interposed between the last entrybridle assembly roller 141 and the additional heated roller 143, isapproximately 219° F., the sheet workpiece 122 obviously having beensubstantially heated by means of the heated entry bridle assemblyrollers 134, 136, 138, 140, and 141. As a result of the further routingof the sheet workpiece 122 about the outer peripheral or circumferentialsurface of the additional heated roller 143, the outer surface of thesheet workpiece 122, that is, the surface of the sheet workpiece 122which will not be disposed in contact with the upper or top roller 142of the zero-gap assembly 120, is heated still further such that at alocation interposed between the additional heated roller 143 and theupper or top roller 142 of the zero-gap assembly 120, the temperature ofthe outer surface of the sheet workpiece 122 is approximately 242° F.This is somewhat higher than the temperature of the opposite or innersurface of the sheet workpiece 122 which was not directly heated bymeans of the additional heated roller 143, however, when the innersurface of the sheet workpiece 122 comes into contact with the upper ortop roller 142 of the zero-gap assembly 120, such inner surface will beheated further. In addition, the outer surface of the sheet workpiece122 tends to cool due to the fact that such surface is not disposed incontact with the heated, upper or top roller 142 of the zero-gapassembly 120 and that such surface is exposed to the ambient air.Consequently, at a location which is immediately upstream of the nip 146defined between the upper and lower rollers 142 and 144, respectively,of the zero-gap assembly 120, both surfaces of the sheet workpiece 122will have approximately the same temperature which is approximately 236°F. In this manner, the density of the material comprising the sheetworkpiece 122 is substantially uniform across the thickness dimension ofthe sheet workpiece 122 whereby optimum properties can be achieved inthe finished sheet 122 which has by such time been simultaneously milledand stretched as a result of being passed through the nip 146 definedbetween the zero-gap assembly rollers 142 and 144. It is to be furthernoted that the outer surface of the finished sheet 122 which has justemerged from the nip 146 defined between the rollers 142 and 144, thatis, the surface of the finished sheet 122 which is not disposed incontact with the lower or bottom roller 144, has a temperature ofapproximately 290° F., the sheet 122 having experienced such a dramaticincrease in its temperature due to the working of the same within thezero-gap assembly 120 and having undergone simultaneous milling andstretching within the zero-gap assembly 120. After being routed aroundthe outer peripheral or circumferential surface of the lower or bottomroller 144 of the zero-gap assembly 120, the finished sheet 122 isconducted downstream for further processing thereof in to elongatedpackaging straps, and at this location, the sheet 122 may exhibit asurface temperature of approximately 277° F.

It is to be emphasized that the importance of the provision of theadditional heated roller 143 is to, in effect, "overheat" the outersurface of the sheet workpiece 122 with respect to the inner surface ofthe sheet workpiece 122 to such a degree that such "overheating" willcompensate for the fact that such outer surface of the sheet workpiece122 will not be otherwise heated as will the inner surface of the sheetworkpiece 122 when the same is disposed in contact with the heated upperroller 142 of the zero-gap assembly 120, and that such outer surface ofthe sheet workpiece 122 will experience cooling thereof, which the innersurface of the sheet workpiece 122 will not, due to the fact that theouter surface of the sheet workpiece 122 is exposed to the ambient airwhereas the inner surface of the sheet workpiece 122 is not exposed tothe ambient air due to its surface contact with the outer peripheralsurface of the upper roller 142 of the zero-gap assembly 120. In view ofthese factors, both surfaces will therefore exhibit substantially thesame surface temperatures at a point immediately upstream of the nip 146defined between the zero-gap assembly rollers 142 and 144 whereby theimproved density profile across or throughout the thickness of the sheetworkpiece 122, and the resultant properties derived from or dependentupon such uniform density profile, are able to be achieved.

Continuing further, it is known that during processing of the sheetworkpiece 122, that is, by means of the aforenoted milling andstretching thereof, the side edge portions of the sheet workpiece 122will tend to be thicker than the more centrally located portions of thesheet workpiece 122. The reason for this is that as the sheet workpiece122 is elongated in the its longitudinal direction, the width andthickness dimensions thereof are accordingly reduced with respect to thewidth and thickness dimensions of the original, non-worked, cast sheetworkpiece 122. In addition, the different regions of the sheet workpiece112 will act differently with respect to each other, thereby leading tothe differences in the thickness dimensions of the edge regions orportions of the sheet workpiece 122 and of the central portions orregions of the sheet workpiece 122. For example, if the sheet workpiece122 was divided across its width dimension into equal small regions, itwould be appreciated that the central regions would be laterallyconfined or restrained by neighboring regions upon opposite sidesthereof, whereas within the edge regions of the sheet workpiece 122,such regions are only confined or restrained in one lateral direction orupon one side thereof because the opposite side comprises a free edge orside. In a similar manner, the upper and lower surfaces of the sheetworkpiece are not confined or restrained, and this is true for both thecentral and edge portions or regions of the sheet workpiece.Consequently, when the sheet workpiece undergoes elongation, the centralregions of the sheet workpiece cannot be reduced in width as much as theedge regions or portions because of the additional lateral constraintsor confinements imposed upon the central regions or portions. However,in order that each region or portion retain its original volumetricvalue, even though the volumetric configuration of a particular regionor portion may be reorganized or rearranged, the thickness dimension ofthe central regions or portions is reduced to a greater extent than thethickness dimension of the edge portions or regions, or alternatively,the thickness dimension of the edge regions or portions is greater thanthe thickness dimension of the central regions or portions of the sheetworkpiece. When the sheet workpieces used in connection with the presentinvention usually have a width dimension of approximately twenty-fourinches (24") after being simultaneously milled and stretched within thezero-gap assembly 120, it has been found that the thicker edge portionsoccur upon the opposite sides of the oriented or finished sheet 122 fora lateral or widthwise extent of approximately one and one-half inches(1.5").

In accordance with one of the objectives of the present invention, it isdesired to effectively reduce the lateral or widthwise extent of thethickened side edge portions or regions of the finished oriented sheet122, and it has been found that if the side edge portions of the sheetworkpiece 122 were heated prior to the entry of the sheet workpiece 122into the nip 146 defined between the zero-gap assembly rollers 142 and144, the lateral or widthwise extent of the thickened side edge portionsor regions of the finished oriented sheet 122 could be dramaticallyreduced whereby a substantially increased volume of the finishedoriented sheet 122, having a desired, uniform thickness or degree offlatness, could be achieved. In particular, it has been found that thelateral or widthwise extent of each thickened side edge portion orregion of the finished oriented sheet 122 can be significantly reducedfrom the aforenoted one and one-half inches (1.5") to approximately onehalf inch (0.5"). This reduction in the lateral or widthwise extent ofthe thickened side edge regions or portions of the finished orientedsheet 122 thus results in less waste or trim to be removed from thefinished oriented sheet 122 and concomitantly results in a larger amountor volume of finished oriented sheet 122 which may then be processedinto strapping material.

Briefly, as best understood, one reason that the aforenoted heating ofthe side edge portions of the sheet workpiece prior to entry into thenip region 146 defined between the zero-gap assembly rollers 142 and 144would result in the reduction of the widthwise or lateral extent of thethickened side edge portions of the finished oriented sheet 122 is thatwithin the sheet workpiece 122, tensile forces or loads act inwardlytoward the central region of the sheet workpiece 122, and the ability toresist tensile loads is a function of temperature. Consequently, whenyou heat the side edge portions of the sheet workpiece 122, theresistance to the tensile loads is decreased within those regions orportions whereby such outer or side edge portions or regions tend toexpand in the lateral or widthwise direction such that the side edgeportions or regions of the finished oriented sheet 122 tend to becomethinner with a somewhat corresponding increase in the overall width ofthe finished oriented sheet 122 as compared to a similarly finished,oriented sheet 122 which was not pre-heated within the side edgeportions or regions thereof.

With reference therefore again being made to FIG. 5, it is seen that inaccordance with the principles of the present invention, andfurthermore, in accordance with the foregoing discussion, the apparatusor system 100 has incorporated therein radiant, infrared heaters whichare adapted to pre-heat the edge portions of the sheet workpiece 122prior to its entry into the nip 146 defined between the upper and lowerzero-gap assembly rollers 142 and 144. In particular, a pair of heaters188 are effectively disposed with respect to the upper zero-gap assemblyroller 142 so as to envelop the same for an arcuate or circumferentialextent of 180°, and the heaters 188 are also axially disposed along theaxis of the roller 142 so as to be optimally positioned with respect tothe oppositely disposed edge portions of the sheet workpiece 122. Eachone of the heaters 188 is approximately two and one-half inches (2.5")wide and is operated at 240 VAC with a generated power of 2500 Watts. Inaddition, a second pair of heaters 190, which are linear inconfiguration, are operatively connected to the upstream ends of theheaters 188, respectively, as viewed in the direction of travel of thesheet workpiece 122 toward the zero-gap assembly 120. These heaters 190are similarly two and one-half inches (2.5") wide, however, they mayonly need to generate a power of 1900 Watts at 240 VAC. Still further,as yet an additional option or alternative, the sheet workpiece 122 maybe initially pre-heated within the entry bridle assembly 132 by means ofthe disposition of additional heaters 192 which are similar to theheaters 188 in that they have the arcuate configuration enveloping, forexample, the upper entry bridle assembly roller 138 over acircumferential extent of 180°, however, such heaters 192 need onlygenerate 1500 Watts power at 240 VAC, but the heaters 192 are of thesame width as that of heaters 188. The edge heaters 188 and 190 are alsoillustrated in FIG. 6.

As has been noted hereinbefore, while the aforenoted edge heaters 188and 190 have in fact been instrumental in reducing the thicknessdimensions of the sheet workpiece 122 at the extreme outer edge portionsthereof, additional studies performed in connection with the strapprofile and the effects upon the same by means of the disposition of theaforenoted edge heaters have revealed that concave portions orconcavities are nevertheless still present within regions immediatelyadjacent to the extreme outer edge portions of the sheets. The concaveportions have depth or thickness dimensions which are somewhat less thanthe thickness dimension characteristic of the main central portion ofthe milled and stretched sheet, and in addition, a transitional bumpregion is also present at the junction between the main central portionof the sheet and the concave side edge portions thereof. Thetransitional bump region has a thickness dimension which is greater thanthat of the main central portion of the sheet.

Consequently, in accordance with further developments attendant theproduction of oriented plastic strap, and particularly in accordancewith the principles of the present invention, it has been determinedthat if additional heaters are disposed in a similar manner with respectto the sheet workpiece 122 as are the edge heaters 188, but disposedaxially inwardly of the heaters 188 so as to be disposed toward the maincentral portion of the sheet workpiece 122, and in particular, disposedat axial positions along the sheet workpiece 122 corresponding to thetransitional bump regions, the latter regions as well as the concaveportions or concavities can be effectively eliminated or substantiallyreduced such that the thickness profile of the milled and stretchedsheet 122 can be substantially more uniform across the entire widthwiseextent thereof.

More particularly, and with reference being made to FIG. 9, across-sectional profile, in effect, is graphically illustrated inconnection with a typical pilot milled and stretched sheet workpiecewhich has a width dimension of four and one-half inches (4.5"). Thegraphical data compiled and graphically illustrated in connection withthe pilot workpiece accurately reflects similar data and profiles whichwould be replicated in connection with actual production sheetworkpieces which have a width dimension of twenty-four inches (24") asnoted hereinbefore. Continuing, therefore, as can be appreciated fromgraphically plotted curve A, which graphically illustrates or representsthe profile of a milled and stretched sheet workpiece when neither theaforenoted edge heaters nor the additional strip heaters of the presentinvention were employed, the sheet workpiece profile is seen to compriseconcave portions or concavities A_(c) upon both edge portions of thesheet workpiece, and the concave portions are connected to the maincentral portion of the sheet workpiece through means of a pair oflaterally spaced bump transition points A_(t). It has been observed thatthe bump transition points A_(t) occur at the one and one-quarter inch(1.25") and three and one-quarter inch (3.25") points along the width ofthe milled and stretched pilot sheet workpiece, and therefore inaccordance with the present invention, strips heaters 194, as seen inFIGS. 7-9, are disposed about the entry roller 142 of the zero-gapassembly 120, in a manner similar to the disposition of the edge heaters188, but at axial positions along the axial length of the sheetworkpiece 122 which correspond to the axial locations of the bumptransition points A_(t), that is, axially inwardly of the edge heaters188. The heaters 194 envelop the upper zero-gap assembly roller 142 overan arcuate or circumferential extent of 180°, and may comprise radiantinfrared heaters similar to the edge heaters 188. The heaters 194 mayalso be adjustably disposed with respect to the sheet workpiece 112 bysuitable adjusting means 196 so as to desirably or optimally vary theeffects of such heaters 194 upon the sheet workpiece 122. The axialextent of the heaters 194 may also be suitably varied, and such spacingand axial length variations affect the width or axial length of theheated areas of the workpiece whereby the infrared radiation can haveincreased or decreased distribution effects upon the sheet workpiece andin particular in connection with the bump transition points A_(t) orregions. Curves B, C, and D graphically illustrate in FIG. 9 actual datacorresponding to the various thickness profiles of milled and stretchedsheet workpieces when heaters 194 having lengths of five-eighths of aninch (0.625"), one inch (1.0"), and one and one-half inches (1.5"),respectively, were employed. It is to be further noted from the graph ofFIG. 9 that for graphical comparison purposes, the effects of the stripheaters 194 have only been graphically illustrated upon the right sideof the graphical profile whereby it can be seen that the profiles of thesheet workpieces have been rendered substantially flatter whereby thebump transition points and the concave portions or concavities have beensubstantially eliminated or significantly reduced. The reason for thisphenomenon is due to be heating of the transitional bump regions, theresultant weakening of such regions with respect to the internal forcespresent within the workpieces and acting thereon as a result of theorientation process, and consequently, the transitional bump regionsexhibit or undergo a reduction in their thickness dimensions while theportions of the workpieces disposed immediately adjacent to suchtransitional bump regions experience or undergo a correspondingenhancement in their thickness dimensions. Roller forces also impressedupon sheet workpieces likewise play a role in such thickness reductionand enhancement characteristics across the widths of the sheetworkpieces. Comparison with the left side of the graph of FIG. 9,wherein the effects of the strip heaters 194 have not been illustrated,clearly disclose the presence of the bump transition points and theconcavities or concave portions. Continuing still further, asadditionally appreciated from the left side of the graph of FIG. 9,curve E represents an anticipated or typical sheet workpiece profilewhen only the edge heaters 188 and 190 would be employed. As can beseen, while the thickness dimension would be reduced, the sheet profilewill still exhibit the bump transition point and the concavity orconcave portion. Yet further, the curve ES represents an anticipated ortypical sheet workpiece profile when both the edge and strip heaterswould be employed. As can be appreciated, the bump transition point willbe effectively eliminated, the concavity or concave portion will also beeffectively eliminated, and the thickness dimension at the edge portionwill be substantially reduced.

Thus, it may be seen that in accordance with the foregoing, andparticularly in accordance with the manufacturing techniques andapparatus developed in connection with the principles of the presentinvention, the finished milled and stretched sheet workpieces exhibitimproved degrees or uniformity of flatness across the entire widthdimension thereof whereby increased volumes of thermoplastic strappingmay be produced or derived therefrom so as to render such manufacturemore cost-effective.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the presentinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be protected by means of LettersPatent of the United States of America, is:
 1. In combination, a systemfor simultaneously milling and stretching a sheet workpiece which passesthrough said system in a predetermined direction of travel, comprising:asheet workpiece; a pair of opposed rollers defining a nip therebetweenand through which said sheet workpiece is passed so as to besimultaneously milled and stretched as said sheet workpiece passesthrough said nip in a single pass; means for driving said opposedrollers in opposite directions and at sufficiently different linealsurface velocities for simultaneously milling and stretching said sheetworkpiece within said nip defined between said pair of opposed rollers;and separate individual means for heating opposite side portions of saidsheet workpiece, as considered in the widthwise extent of said sheetworkpiece, upstream of said nip, as considered along said predetermineddirection of travel of said sheet workpiece, wherein said nip is definedbetween said pair of opposed rollers, and at axial positions across thewidth of said sheet workpiece which are located predetermined distancesaxially inwardly from opposite side edge portions of said sheetworkpiece so as to increase the degree of flatness of saidsimultaneously milled and stretched sheet workpiece across said width ofsaid simultaneously milled and stretched sheet workpiece after saidsimultaneously milled and stretched sheet workpiece has passed throughsaid nip defined between said pair of opposed rollers by eliminating anyconcave portions and bump transitional points defined between saidconcave portions and a main central portion of said sheet workpiecewhich tend to form at and within the vicinity of said axial positionslocated said predetermined distances axially inwardly from said oppositeside edge portions of said sheet work-piece.
 2. The combination as setforth in claim 1, wherein:said heating means comprises radiant, infraredheaters.
 3. The combination as set forth in claim 1, wherein:saidheating means partially envelop a first upstream one of said pair ofopposed rollers as considered in said direction of travel of said sheetworkpiece through said system.
 4. The combination as set forth in claim2, wherein:each one of said radiant infrared heaters has an axiallength, oriented across said width of said sheet workpiece, which iswithin the range of 0.625-1.5 inches.
 5. The combination as set forth inclaim 1, further comprising:additional means for heating said oppositeside edge portions of said sheet workpiece at a location upstream ofsaid nip defined between said pair of opposed rollers so as to cooperatewith said means for heating said opposite side portions of said sheetworkpiece so as to achieve said degree of flatness of saidsimultaneously milled and stretched sheet workpiece across said width ofsaid simultaneously milled and stretched sheet workpiece.
 6. Thecombination as set forth in claim 5, wherein:said additional heatingmeans comprises radiant, infrared heaters.
 7. The combination as setforth in claim 1, wherein:said predetermined distances axially inwardlyfrom said opposite side edge portions of said sheet workpiece, at whichsaid heating means are located, are approximately one and one-quarterinches (1.25").
 8. Apparatus as set forth in claim 1, wherein:said sheetworkpiece is fabricated from polypropylene.
 9. Apparatus as set forth inclaim 1, wherein:said sheet workpiece is fabricated from polyester. 10.Apparatus for simultaneously milling and stretching a sheet workpiecewhich passes through said apparatus in a predetermined direction oftravel, comprising:a pair of opposed rollers defining a nip therebetweenand through which said sheet workpiece is passed so as to besimultaneously milled and stretched as said sheet workpiece passesthrough said nip in a single pass; means for driving said opposedrollers in opposite directions and at sufficiently different linealsurface velocities for simultaneously milling and stretching said sheetworkpiece within said nip defined between said pair of opposed rollers;and separate individual means for heating opposite side portions of saidsheet workpiece, as considered in the widthwise extent of said sheetworkpiece, upstream of said nip, as considered along said predetermineddirection of travel of said sheet workpiece, wherein said nip is definedbetween said pair of opposed rollers, and at axial positions across thewidth of said sheet workpiece which are located predetermined distancesaxially inwardly from opposite side edge portions of said sheetworkpiece so as to increase the degree of flatness of saidsimultaneously milled and stretched sheet workpiece across said width ofsaid simultaneously milled and stretched sheet workpiece after saidsimultaneously milled and stretched sheet workpiece has passed throughsaid nip defined between said pair of opposed rollers by eliminating anyconcave portions and bump transitional points defined between saidconcave portions and a main central portion of said sheet workpiecewhich tend to form at and within the vicinity of said axial positionslocated said predetermined distances axially inwardly from said oppositeside edge portions of said sheet work-piece.
 11. Apparatus as set forthin claim 10, wherein:said heating means comprises radiant, infraredheaters.
 12. Apparatus as set forth in claim 10, wherein:said heatingmeans partially envelop a first upstream one said pair of opposedrollers as considered in said direction of ravel of said sheet workpiecethrough said apparatus.
 13. Apparatus as set forth in claim 11,wherein:each one of said radiant infrared heaters has an axial length,oriented across said width of said sheet workpiece, which is within therange of 0.625-1.5 inches.
 14. Apparatus as set forth in claim 10,further comprising:additional means for heating said opposite side edgeportions of said sheet workpiece at a location upstream of said nipdefined between said pair of opposed rollers so as to cooperate withsaid means for heating said opposite side portions of said sheetworkpiece so as to achieve said degree of flatness of saidsimultaneously milled and stretched sheet workpiece across said width ofsaid simultaneously milled and stretched sheet workpiece.
 15. Apparatusas set forth in claim 14, wherein:said additional heating meanscomprises radiant, infrared heaters.
 16. Apparatus as set forth in claim10, wherein:said predetermined distances from said opposite side edgeportions of said sheet workpiece, at which said heating means arelocated, are approximately one and one-quarter inches (1.25"). 17.Apparatus as set forth in claim 9, wherein:said polyester comprisespolyethylene terephthalate.